1. Field of the Invention
The present invention relates to a power supply unit for an electronic device such as an electronic device having a tuner.
2. Description of the Related Art
Generally, an electronic device such as a television receiver, to be connected to an AC power supply in use, is internally provided with a power supply unit such as a switching power supply to convert a voltage from the AC power supply to multiple different voltages for output. In the case of a television receiver, an internal switching power supply, for example, is provided to supply multiple kinds of voltages such as a relatively high voltage output (e.g. 33 volts) for operating a tuner and a relatively low voltage output (e.g. 7 volts) for operating a microcomputer. This will be described below with reference to FIG. 2.
FIG. 2 is a schematic block diagram of a conventional power supply unit Pc which is used for a television receiver as an electronic device. Referring to FIG. 2, the conventional power supply unit Pc, which has a DC power supply circuit 103 with multiple voltage output terminals, will be described. The power supply unit Pc comprises a microcomputer 107, a control circuit 106, a DC power supply circuit 103, an on/off controllable four-terminal regulator (high voltage) 108 and an on/off controllable four-terminal regulator (low voltage) 109 as well as regulators attached to the microcomputer 107, on/off controllers operated by the microcomputer 107, and diodes as shown. The DC power supply circuit 103 has two voltage output terminals, i.e. a high voltage terminal 101 for outputting a relatively high voltage V1 and a low voltage terminal 102 for outputting a relatively low voltage V2. The high voltage terminal 101 is connected via a diode to a television tuner 104 of a television receiver (not entirely shown), while the low voltage terminal 102 is connected via a diode to a small load circuit 105 (e.g. a lamp circuit of the television receiver). An AC power source is connected to the primary side of the DC power supply circuit 103.
The DC power supply circuit 103 is connected to the microcomputer 107 via the control circuit 106 which serves as a driver circuit. When a user switches the operation mode of the television receiver from normal power-on (power supply-on) mode to standby (power standby) mode, the microcomputer 107 outputs a voltage reduction command to the control circuit 106 so as to reduce the output voltages V1, V2 of the high voltage terminal 101 and low voltage terminal 102, respectively. Specifically, the output voltages V1 and V2 from the high voltage terminal 101 and low voltage terminal 102 are set to be 33 volts and 8 volts, respectively, when the television receiver is in the power-on mode. When the television receiver is switched to the standby mode, the output voltages from the high voltage terminal 101 and low voltage terminal 102 are reduced to 8 volts and 4 volts, respectively. The standby mode thus allows the power consumption to be significantly saved. Furthermore, the output from the high voltage terminal 101 is connected to the microcomputer 107 via the on/off controllable regulator 108, while the output from the low voltage terminal 102 is connected to the microcomputer 107 via the on/off controllable regulator 109.
For operating the microcomputer 107 in the power supply unit Pc, specifically, the voltage of a voltage supply point (point A) to the microcomputer 107 is to be at least 7 volts. In order to provide the voltage of the voltage supply point (point A), which is to be supplied to the regulators attached to the microcomputer 107, the output voltage from the regulator 108 connected to the high voltage terminal 101 and the output voltage from the regulator 109 connected to the low voltage terminal 102 are switched by controlling the on/off switching of the regulators 108, 109. Note that the regulators 108, 109 are four-terminal on/off controllable regulators, which, when turned on, reduce the voltages from the high voltage terminal 101 and low voltage terminal 102 to 7 volts (the same voltage) so as to output this voltage of 7 volts, and which, when turned off, stop output.
More specifically, when the operation mode of the television receiver is power-on mode, the regulator 108 is switched off while the regulator 109 is switched on, so as to reduce the voltage V2 (8 volts) from the low voltage terminal 102 to 7 volts, which is then supplied to the voltage supply point (point A) to supply the voltage (7 volts) to the regulators attached to the microcomputer 107. On the other hand, when the operation mode is standby mode, the regulator 108 is switched on while the regulator 109 is switched off, so as to reduce the voltage V1 (8 volts) from the high voltage terminal 101 to 7 volts, which is then supplied to the voltage supply point (point A) to supply the voltage (7 volts) to the regulators attached to the microcomputer 107. Thus, the conventional power supply unit Pc is advantageous in that regardless of the operation mode, the input voltage (8 volts) to the regulators 108, 109 is close to the output voltage (7 volts) from the regulators 108, 109, so that the heat loss in the regulators 108, 109 can be reduced. However, the conventional power supply unit Pc at the same time has problems as described below.
That is, there is a risk that there may be a moment when both regulators 108, 109 are in the off state because the timing of switching the regulator 108 on/off is not synchronized with the timing of switching the regulator 109 on/off at such moment. In this case, there is a risk that the supply voltages to the microcomputer 107 may become lower than a minimum required voltage, thereby causing a malfunction. Another problem is that it is required to install, in the microcomputer 107, a control program for the on/off control of the regulators 108, 109. At the same time, the microcomputer 107 is required to have ports for outputting on/off control signals to the regulators 108, 109. This makes it necessary to provide signal lines to be connected to these ports. This is also a problem because it reduces the degree of freedom in circuit design.
Technologies relating to power supply units in this field include those described below. For example, in the electronic device with a power supply unit as described above, a malfunction is caused when the timing of the voltage supply to the microcomputer is not synchronized with the timing of the voltage supply to circuits other than the microcomputer. In order to allow the timing of the voltage supply to the microcomputer to be accurately synchronized with the timing of the voltage supply to other circuits than the microcomputer, a known malfunction prevention device is designed such that multiple voltages output from a power supply unit are converted to the same voltage via a regulator, and are logically summed by a logic circuit, the output of which is input to the microcomputer (refer to e.g. Japanese Laid-open Patent Publication Hei 5-143202).
There is also a system consisting of a modem with a built-in single chip CPU (Central Processing Unit) connected to an external data communication terminal which supplies a voltage to the modem. The modem has a regulator circuit and a DC-DC converter circuit as well as a voltage determination circuit therein for selecting either the regulator circuit or the DC-DC converter circuit according to the kind of voltage supplied from the data communication terminal, so as to form a voltage conversion circuit to supply a predetermined voltage to the single chip CPU through selected one of the regulator circuit and the DC-DC converter circuit (refer to e.g. Japanese Laid-open Patent Publication Hei 8-22335). The technologies described in these patent documents, however, cannot solve the problems as described above.